one who organizes, manages, and assumes the risks of a business or enterprise

The one that resonates most with me is the one coined by HBS professor Howard H. Stevenson:

The pursuit of opportunity beyond the resources you currently control.

So why do I like this definition? Because it rings more true of my experience than any of the other definitions. It does so in the following ways:

It actually is a pursuit. You have to wake up every day and make things happen. It doesn't happen to you by chance. If you're a one-man-band, sometimes this can be difficult -- but you have to persist and keep moving things along.

Of course, there is opportunity -- something you've spotted that you think has not been taken advantage of. It's an opportunity you think will change the world, or at least your bank balance.

As may be apparent from other posts, I like the resource-centric perspective of how businesses are organized. Any business is a configuration of resources.

The key part, however, is the phrase "beyond the resources you currently control". Everything you are trying to do is through leveraging resources that you don't own. If you had complete control to everything you needed, it would be easy. Since you do not, I believe the real skill in entrepreneurship is in figuring out ways to access and use other peoples' resources -- other peoples' time, money, relationships or whatever else you may need to build your business.

This is just my perspective. You may better enjoy Stevenson's far more formal explanation to this definition of entrepreneurship:

The first attribute is pursuit of opportunity. Strategic orientation is driven either by identifying new opportunities or by the resources you currently control. The second is quick commitment—getting there fast versus developing a yearlong strategic plan. The third is a multistaged commitment process, which is best described as the difference between a fighter bomber and a ballistic missile. The fighter bomber can take evasive action, choose its targets of opportunity, and sometimes just cut and run when the odds are overwhelming. The ballistic missile is carefully aimed to arrive on target; it's going to follow that course even with a Scud missile approaching. The fourth attribute, the use of others' resources, contrasts with the idea that you ought to own or employ everything you need. For example, good entrepreneurial firms often hire the most specialized talent on a temporary basis. The fifth, managing through networked relationships, contrasts with the old paradigm that "management is what you do to the people who work for you." The final attribute is rewards based on value created rather than position in a hierarchy.